Signal transduction is any process by which a cell converts one kind of signal or stimulus into another. Processes referred to as signal transduction often involve a sequence of biochemical reactions inside the cell, which are carried out by enzymes and linked through second messengers. Signal transduction is often accomplished by the activation of enzymes that can act upon other enzymes and change their catalytic activity. This may lead to increases or decreases in the activity of certain metabolic pathways, or may lead to even large intracellular changes, for example, the initiation of specific patterns of gene expression and/or changes in cell proliferation.
The most common covalent modification used in signal transduction processes is phosphorylation, which results in the alteration of the activity of those enzymes which become phosphorylated. Phosphorylation is the addition of a phosphate (PO4) group to a protein or a small molecule. Any of several amino acids in a protein may be phosphorylated. Phosphorylation on serine is the most common, followed by threonine. Tyrosine phosphorylation is relatively rare. However, since tyrosine phosphorylated proteins are relatively easy to purify using antibodies, tyrosine phosphorylation sites are relatively well understood. Histidine and aspartate phosphorylation occurs in prokaryotes as part of two-component signaling. Other types of phosphorylation include oxidative phosphorylation. Adenosine triphosphate (ATP), the “high-energy” exchange medium in the cell, is synthesized in the mitochondrion by addition of a third phosphate group to Adenosine diphosphate (ADP) in a process referred to as oxidative phosphorylation. ATP is also synthesized by substrate level phosphorylation during glycolysis. ATP is synthesized at the expense of solar energy by photophosphorylation in the chloroplasts of plant cells.
In eukaryotes, protein phosphorylation is probably the most important regulatory event. Many enzymes and receptors are switched “on” or “off” by phosphorylation and dephosphorylation. Phosphorylation is catalyzed by enzymes known as ATP-dependent phosphotransferases which are often simply referred to as “kinases.” These include, among others, protein kinases, lipid kinases, inositol kinases, non-classical protein kinases, histidine kinases, aspartyl kinases, nucleoside kinases, and polynucleotide kinases.
Phosphorylation regulates protein function, for example, by affecting conformation. This in turn regulates such processes as enzyme activity, protein-protein interactions, subcellular distribution, and stability and degradation. The stoichiometry of phosphorylation of a given site is controlled by the relative activities of a cell's repertoire of protein kinases and phosphatases. Thus phosphorylation can often generate extremely rapid and reversible changes in the activity of target proteins. The ability to assay the state of phosphorylation of specific proteins is of great utility in the quest to establish the function of a given protein. Such assays are also critical for the identification of drugs that can influence the phosphorylation, and hence the function, of specific proteins.
In general, phosphoproteins are highly unstable and difficult to produce, both in terms of specific phosphorylation of biologically relevant amino acids and subsequent purification of protein. A means to specify and drive a targeted phosphorylation event with a high degree of certainty and efficiency is needed. This is particularly important for recombinant proteins expressed in bacterial or fungal expression systems which do not phosphorylate proteins in the same way as mammalian cells.
Therefore, it is an object of the present invention to provide a method for the site specific phosphorylation of proteins.
It is further an object of the present invention to provide a method for the site specific phosphorylation of proteins in vivo.
In particular, it is an object of the present invention to provide a method for the site specific incorporation of phosphoserine into a protein.